The anticipated advantages of arc-discharge automotive headlamps are an improved roadway illumination pattern by comparison with conventional filament headlamps; lower and shorter headlamp reflectors which permit improvements in aerodynamic efficiency and greater freedom in automotive styling, e.g. lowering the hood line; longer-lived lamps lasting the life of the vehicle; and lower consumption of energy which saves fuel and also reduces the thermal load on the plastic components of the headlamp housing.
An essential requirement of an automotive headlamp is ability to provide light immediately when needed, including instant relight after a momentary turn-off. U.S. patent application Ser. No. 157,436 filed Feb. 18, 1988 by Rolf S. Bergman et al., titled Xenon-Metal Halide Lamp Particularly Suited for Automotive Applications, assigned to the same assignee as the present invention, discloses a xenon-metal halide discharge lamp which combines the high efficacy and long life of the metal halide lamp with the instant-light capability of a xenon arc tube. Thus the Bergman lamp satisfies the main requirements of a discharge headlamp.
The presence of high pressures of xenon in a xenon-metal halide lamp appreciably increases gravity-induced convection, and this tends to limit the benefits achieved. Convection in the fill causes undesirable results: (1), upward bowing of a horizontal arc, while lamp optics require a straight line; (2), higher temperature of the hot spot at the envelope wall above the arc, a condition which tends to shorten lamp life; and (3), lower temperature of the cold spot at the wall under the arc, a condition which reduces lamp efficacy.
An arc may be stabilized by heat loss to the electrodes known as electrode stabilization, or to the walls, known as wall stabilization, or to both, and these are the ordinary conventional ways of overcoming convection effects. Typical instances of stabilization almost entirely by the walls are the ordinary fluorescent lamp wherein a low intensity arc through mercury vapor is confined in a 1.5 inch diameter glass tube up to 8 feet long, and the high pressure sodium street lamp wherein an intense arc through sodium vapor is confined in a 3/16 inch diameter alumina ceramic tube 10 cm. long. An example of electrode stabilization is the high pressure xenon arc lamp used for photography and signalling.
When the arc in a xenon-metal halide arc tube for a vehicle headlamp is stabilized in conventional ways, the need to avoid excessive heat load on the quartz arc tube imposes limits. With wall stabilization, the length of the arc and the dimensions of the enclosing tube must be made almost equal in size to the filament source it is replacing, so that the anticipated advantages of reduced size are lost. With electrode stabilization, the arc tube can be used as a headlamp source if the gap is kept very short in order to reduce bowing. By way of example, these restrictions may limit source efficacy to 35-40 lumens per watt in a typical 40 watt design. While this is 2-3 times the efficacy of an arc tube filled with xenon alone, it falls well short of the 70 to 80 lpw efficacy typically achievable with miniature high pressure metal halide lamps not containing xenon gas, for instance lamps as in U.S. Pat. No. 4,161,672 Cap and Lake.
In high pressure metal vapor lamps, deionization is more rapid when the size of the lamp is reduced, and this raises the reignition voltage. Cataphoresis of metal atoms in a discharge lamp is the concentration of those atoms about the cathode, resulting in their depletion from the arc and a reduction in generated radiation and/or a change in color. Because miniature metal halide lamps are subject to cataphoresis of the metal atoms under d.c. and low frequency operation (up to about 1 kHz), and also because they have unacceptably high reignition voltages at frequencies below 100 Hz, operation at higher frequencies is desirable. Higher frequency operation also results in improved efficacy since the gas discharge does not have time to cool off appreciably between half-cycles of current.